One of my cousins has her kids enrolled in what appears to be a pee-wee karate class. In one of the photos it shows a kid proudly displaying his black belt. The kid looks to be no more than five or six years old. When I was broadening my martial horizons, I cross-trained in a fair number of styles. The one thing that always held true across all of them was that a black belt (or its equivalent) is representative of some mastery of that system; it means you can lead and teach a class yourself. There was a Shotokan school in Charlottesville that had a really excellent method for handling teaching kids. For 7 year old kids and younger, the kids got a colored belt with a long white stripe down the center. The kids moved up in ranks right up until they hit brown belt with a white stripe. Once they were older than 7, if they had progressed to brown/white belt than they moved directly to an orange belt (the progression in Shotokan is white, orange, green, purple, brown, black). If they came in older than 7 than they started at white belt and progressed as normal. If this were a game you would call that a very balanced leveling system. I liked it because it gave the kids a feeling of accomplishment without providing false expectations about their skills.